The neocortex, a structure covering mammalian brains, computes high-order sensory, motor, and cognitive processes. Over the course of evolution, the neocortex of certain species expanded dramatically and folded, thereby providing superior sensorimotor and cognitive abilities. The immense expansion of the neocortex in humans has made possible the complex behavior, cognition, and intellect that are unique to humans. Neocortical expansion and folding reflect an increase in the number of neural cells and are thus dependent on the number of neural progenitor/stem cells. The long-term goal of this project is to understand the mechanisms regulating neural progenitor/stem cell expansion and the growth and folding of the neocortex. Sonic hedgehog (Shh) signaling promotes the expansion of neural progenitor/stem cells, leading to the growth and folding of the neocortex in mice. Shh signaling also increases the number of neural progenitor/stem cells in human cerebral organoids, a miniature model of the developing brain grown from human pluripotent stem cells. The short-term goal of this project is to understand how Shh signaling expands neural progenitor/stem cells. Shh signaling controls the expression of a set of genes (Shh effectors) to exert its biological activity. This project has two main components: 1) the expression of Shh effectors will be artificially increased or decreased in neural progenitor/stem cells in mice and human cerebral organoids in order to understand how Shh effectors regulate neural progenitor/stem cells; and 2) the strength of Shh signaling will be increased or decreased in ferret fetuses and ferret brain slices to investigate whether and how Shh signaling regulates neural progenitor/stem cells in intact animals with folded brains. The proposed research will provide deeper insights into the mechanisms underlying neocortical expansion and folding and the etiology of neurodevelopmental diseases.